dimmer: tentative to quit if we are obscured (presumably by i3lock)

This does not happend because, I suppose, we are using a compositor.

bin/dimmer

@@ -13,9 +13,15 @@ import cairo
 import argparse
 
 
+def visibility_event(widget, event):
+    if event.state == Gdk.VisibilityState.FULLY_OBSCURED:
+        gtk.main_quit()
+
+
 def realize_event(widget):
     window = widget.get_window()
     window.set_override_redirect(True)
+    window.set_events(window.get_events() | Gdk.EventMask.VISIBILITY_NOTIFY_MASK)
 
 
 def draw_event(widget, event, options, elapsed):
@@ -82,8 +88,9 @@ if __name__ == "__main__":
         window.move(geom.x, geom.y)
 
         window.connect("draw", draw_event, options, [])
-        window.connect("delete-event", Gtk.main_quit)
         window.connect("realize", realize_event)
+        window.connect("visibility-notify-event", visibility_event)
+        window.connect("delete-event", Gtk.main_quit)
 
         window.show_all()